As a heat exchanger to be used in an air-conditioning apparatus such as a room air-conditioning apparatus and a package air-conditioning apparatus, a finless heat exchanger and a fin-and-tube heat exchanger have been known. In the fin-and-tube heat exchanger using pipes and fins, thermal contact resistance is generated between the pipe and the fin, and resistance is also generated in a fin portion due to thermal conduction of the fins. In contrast, the finless heat exchanger does not include fins, and hence resistance due to thermal conduction of the fins is almost zero. Further, thermal contact resistance generated between the pipe and the fin is also zero. Consequently, the heat exchanger performance is enhanced. Further, when the finless heat exchanger is used as an evaporator, condensed water flows downward through spaces defined by flat pipes while meandering in the gravity direction. Consequently, the drainage performance is satisfactory. Further, in a case in which the finless heat exchanger is used as a heat exchanger of an outdoor unit, also at the time of a defrosting operation after an operation causing frost formation, accumulation of ice on a lower part of the heat exchanger can be prevented.
As the finless heat exchanger to be used in the air-conditioning apparatus, for example, Patent Literature 1 below has the following configuration. Specifically, a plurality of flat-shaped heat transfer pipes each accommodating a plurality of passages are arrayed at a predetermined pitch in a direction orthogonal to an air-passing direction so that flat surfaces of the heat transfer pipes are parallel to the air-passing direction. Both ends of the heat transfer pipes are connected to an inlet header and an outlet header. The heat exchanger has a configuration in which an expansion valve is provided in the inlet header to improve refrigerant distribution so that the surface areas of all the flat pipes (heat transfer area) are utilized effectively without waste, thereby the heat exchange performance can be improved.